1. Field of the Invention
The present invention relates to a droplet forming apparatus.
2. Description of the Related Art
Recently, in accordance with a development of a stem cell technology, a technique has been developed in which an organizational body is formed by discharging a plurality of cells by inkjet. As the types of inkjet, a piezoelectric pressure type using a piezoelectric element, a thermal type using a heater, an electrostatic type in which liquid is attracted by an electrostatic attraction or the like may be raised. Among these, it is preferable to use the piezoelectric pressure type for forming a droplet of cell suspension because damages due to heat or an electrical field are harder to be caused to cells by this type compared with other types.
In an inkjet head of a conventional general piezoelectric pressure type, a droplet is formed using compression of liquid in a pressure liquid chamber. Thus, there has been a problem that, if bubbles are mixed in the pressure liquid chamber, the liquid cannot be compressed and the liquid cannot be discharged. For the cell suspension, water is used as solvent. However, a surface active agent that is generally used in a general inkjet ink cannot be used because the surface active agent may cause damages to the cells. Thus, there is a problem that the bubbles are easily mixed in the pressure liquid chamber due to its high surface tension.
Further, in a general inkjet head, in order to remove the bubbles and recover to a normal state, the bubbles are removed with large amount of liquid from a nozzle portion by pressurizing the liquid chamber, or aspirating the liquid from the nozzle portion. However, as the cell suspension is more expensive and valuable compared with general inkjet ink, it is not preferable to remove the bubbles by this method.
Meanwhile, a droplet manufacturing apparatus is disclosed in which liquid on a film is atomized by oscillating the film by a bending mode actuator. In this apparatus, it is possible to directly disperse the liquid formed on the film without using a pressurizing force in the liquid chamber. Thus, compared with a general inkjet head, influence of the bubbles can be reduced (see Patent Document 1, for example).
However, when such an apparatus as described above is used for forming a droplet of cell suspension, as the specific frequency of the film shifts due to the existence of the bubbles remaining in the liquid chamber, there is influence in a droplet forming state if the bubbles of more than or equal to a predetermined amount are mixed. Thus, it is difficult to stably discharge the cell suspension for a long period.